Internal Breaker for Oilfield Treatments

ABSTRACT

A composition and method for improving the fluid efficiency of many oilfield treatments is given. The composition is a solid additive, in a viscosified fluid, in a size range small enough that it enters formation pores; it optionally bridges there to form an internal filter cake, and then decomposes to provide a breaker for the viscosifying system for the fluid. Examples of suitable additives include waxes, polyesters, polycarbonates, polyacetals, polymelamines, polyvinyl chlorides, and polyvinyl acetates. Degradation of the additive may be accelerated or delayed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-part of, and claims the benefitof, U.S. patent application Ser. No. 11/159,023, filed Jun. 22, 2005,which was a Continuation-in-part of, and claimed the benefit of, U.S.patent application Ser. No. 10/707,022, filed Nov. 14, 2003, which was aContinuation-in-part of, and claimed the benefit of, U.S. patentapplication Ser. No. 10/227,690, filed on Aug. 20, 2002, now U.S. Pat.No. 6,938,693. This application is also a Continuation-in-part of, andclaims the benefit of, U.S. patent application Ser. No. 11/316,021,filed Dec. 21, 2005. This application is related to copendingapplication “Internal Breaker for Oilfield Fluids,” inventors PhilipSullivan, Yiyan Chen, Belgin Baser, Carlos Abad, Mehmet Parlar, andGregory Kubala, filed Sep. 18, 2006. This application is also related tocopending application “Oxidative Internal Breaker for ViscoelasticSurfactant Fluids,” inventors Lijun Lin, Carlos Abad, Belgin Baser,Philip Sullivan, Yiyan Chen and Jesse Lee, filed Sep. 18, 2006. Thisapplication is also related to copending application “Method forLimiting Leakoff and Damage in Hydraulic Fractures,” inventors RichardHutchins, Marie Dessinges, and Carlos Abad, filed Sep. 18, 2006. Theseapplications are all assigned to the assignee of the present applicationand are hereby incorporated in their entirety.

BACKGROUND OF THE INVENTION

The Invention relates to recovery of oil and gas from wells, and moreparticularly to decreasing fluid loss and to breaking fluids insideformation pores when using viscoelastic surfactant fluid systems (VES's)as carrier fluids and treatment fluids.

There are many oilfield applications in which filter cakes are needed inthe wellbore, in the near-wellbore region or in one or more strata ofthe formation. Such applications are those in which, without a filtercake, fluid would leak off into porous rock at an undesirable rateduring a well treatment. Such treatments include drilling, drill-in,completion, stimulation (for example, hydraulic fracturing or matrixdissolution), sand control (for example gravel packing, frac-packing,and sand consolidation), diversion, scale control, water control, andothers. Typically, after these treatments have been completed thecontinued presence of the filter cake is undesirable or unacceptable. Insuch oilfield operations as hydraulic fracturing and gravel packing,viscoelastic surfactant (VES) fluid systems are popular as carrierfluids because of their ability to create a very clean proppant orgravel pack. However, they sometimes experience undesirably high fluidloss, especially when formations with permeabilities greater than about5 mD are treated. Consequently, fluid loss additives (FLA's) are oftenused with such carrier fluids to reduce leak off.

There are also many applications in which breakers are needed todecrease the viscosity of treatment fluids, such as fracturing, gravelpacking, and acidizing fluids. Most commonly, these breakers act influids that are in wellbores or fractures; some breakers can work influids in formation pores. Breakers decrease viscosity by degradingpolymers or crosslinks when the viscosifiers are polymers or crosslinkedpolymers. Breakers decrease viscosity by degrading surfactants orchanging or destroying micellar structure when viscosifiers areviscoelastic surfactant fluid systems.

Solid, insoluble materials, such as mica, (that may be called fluid lossadditives (FLA's), lost circulation additives, and filter cakecomponents) are typically added to fluids used in certain treatments toform filter cakes when they are needed, although sometimes soluble (orat least highly dispersed) components of the treatment fluids themselves(such as polymers or crosslinked polymers) may form the filter cakes,provided that the polymers or crosslinked polymers are too large, orrock pores are too small, to permit entry of much of the polymer orcrosslinked polymer. This filter cake is typically on a surface, such asa fracture face. Removal of the filter cake is typically accomplishedeither by mechanical means (scraping, jetting, or the like), bysubsequent addition of a fluid containing an agent (such as an acid, abase, or an enzyme) that dissolves at least a portion of the filtercake, or by manipulation of the physical state of the filter cake (byemulsion inversion, for example). These removal methods usually requirea tool or addition of another fluid (for example to change the pH or toadd a chemical). This can sometimes be done in the wellbore but normallycannot be done in a proppant or gravel pack. Sometimes the operator mayrely on the flow of produced fluids (which will be in the oppositedirection from the flow of the fluid when the filter cake was laid down)to loosen the filter cake or to dissolve the filter cake (for example ifit is a soluble salt). However, these methods require fluid flow andoften result in slow or incomplete filter cake removal. Sometimes abreaker may be incorporated in the filter cake but these must normallybe delayed (for example by esterification or encapsulation) and they areoften expensive and/or difficult to place and/or difficult to trigger.

There would sometimes be advantages to forming a filter cake inside thepores of a formation. For example, such an “internal” filter cake wouldnot be subject to erosion by fluids flowing across a filter cake thatwas formed on a wellbore face, a screen, a fracture face, or similarlocation. Also, an internal filter cake could be more effective atreducing “spurt” the initial fluid loss that occurs as a filter cake isbeing formed. However, formation of internal filter cakes is usuallyavoided, since in the past they have been difficult, if not impossible,to remove.

There is sometimes a need to break viscous fluids within the pores offormations, for example when viscous fluids enter formations duringfracturing, gravel packing, acidizing, lost circulation treatments,scale squeezes, and the like. These fluids that enter the formation maybe main treatment fluids (such as fracturing fluids) or they may besecondary fluids (such as diversion fluids or viscoelastic divertingacids). Most breakers are solids, for example granules or encapsulatedmaterials, that do not enter the formation.

The use of a hydrolysable polyester material for use as an FLA forconventional fluid loss control has previously been proposed; further,degradation products of such materials have been shown to cause delayedbreaking of fracturing fluids. U.S. Pat. No. 4,715,967 discloses the useof polyglycolic acid (PGA) as a fluid loss additive to temporarilyreduce the permeability of a formation. SPE paper 18211 discloses theuse of polyglycolic acid (PGA) as a fluid loss additive and gel breakerfor crosslinked hydroxypropyl guar fluids. U.S. Pat. No. 6,509,301describes the use of acid forming compounds such as PGA as delayedbreakers of surfactant-based vesicle fluids, such as those formed fromthe zwitterionic material lecithin. The preferred pH of the fluid inwhich these materials are used is above 6.5, more preferably between 7.5and 9.5. The use of such materials as destroyable internal filter cakesis also known. U.S. Patent Application Publication No. 2005-0252659,assigned to the assignee of the present application and herebyincorporated in its entirety, describes a method in which the fluidcontains a solid hydrolysable polyacid that upon dissolution andhydrolysis releases an acid that is a breaker for the viscosifyingsystem. Suitable solid hydrolysable polyacids include polylactic acidand polyglycolic acid. The fluid in that Patent Application alsocontains a pH control agent, present in an amount sufficient toneutralize any acid present in the solid hydrolysable polyacid beforethe injection and to neutralize any acid generated by the solidhydrolysable polyacid during the injection, so that the acid breaker isnot available to break the fluid during the injection. In one embodimentthe viscosifier is a viscoelastic surfactant fluid system and the solidhydrolysable polyacid is of a size selected to be a fluid loss additive,for example in fracturing or gravel packing. In another embodiment, thesolid hydrolysable polyacid is used in particles sufficiently small thatthey enter the pores of the formation. In either case, the viscosifieris broken after the solid releases more acid than can be neutralized bythe pH control agent. U.S. Patent Application Publication No.2006-0157248, assigned to the assignee of the present application andhereby incorporated in its entirety, describes an oilfield treatment inwhich particles of a polyol such as polyvinyl alcohol are injected intoa well. The particles are initially insoluble and are made soluble byheating or by a decrease in salinity. The particles may form an internalfilter cake, and after solubilization, they may be breakers.

Additional compositions and treatment methods in which an internalfilter cake is formed, and then the filter cake and the viscosifiersused in the treatment are destroyed, would be of value. It would bedesirable to have a number of materials that are available in smallparticles that could be used in a treatment fluid to form an internalfilter cake in a formation and that then would decompose withoutmechanical or chemical action by the operator so that the filter cakewas destroyed, releasing degradation products that break viscoelasticsurfactant-based viscosifiers in the fluid. It would also be desirableif such materials could be used in particles so small that they did notform a filter cake but still acted as breakers. It would be desirable tohave a number of such materials so that they could be used underdifferent subterranean conditions, for example temperature and formationfluid chemistry.

SUMMARY OF THE INVENTION

A first embodiment of the Invention is a method of treating asubterranean formation penetrated by a wellbore involving a) injectinginto the formation through the formation surface a fluid containing aviscosifier and a solid additive having particles sufficiently small topass into the formation pores. The solid additive includes a materialthat is degradable in the formation, and the additive is allowed todegrade into a material that is soluble in a fluid in the pores afterthe injection. The material is degradable, for example, by a mechanismselected from thermal degradation, melting, hydrolysis and anycombination of these mechanisms. In another embodiment, the fluid isallowed to flow between the formation and the wellbore or between thewellbore and the formation.

In another embodiment, the particles may form an internal filter cake,that is, a filter cake inside the pores of the matrix. In yet anotherembodiment, the particles may optionally form an external filter cake,that is, a filter cake on the face of the formation.

Various other embodiments involve one or more of the following aspects:The soluble material is a breaker for the viscosifier in the fluid. Thestep of injection is optionally carried out above the formation fracturepressure to form one or more fractures. The fluid carries proppant orgravel. The fluid contains a formation dissolving compound. Theviscosifier is a viscoelastic surfactant, for example a quaternaryamine, betaine, carboxylic acids, or amidoamine oxide. The viscosifieris a polysaccharide.

In another embodiment, the viscosifier is a polysaccharide and theparticles include a material selected from polyesters, polycarbonates,polyacetals, polymelamines, polyvinyl chlorides, polyvinyl acetates,polyvinyl esters, polyacrylates, polymethacrylates, polycarboxylates andmixtures of these materials.

In another embodiment, the particles include a material selected fromwaxes, polycarbonates, polyacetals, polymelamines, polyvinyl chlorides,polyvinyl acetates, polyvinyl esters, polyacrylates, polymethacrylates,polycarboxylates, and mixtures of these materials.

In another embodiment, the fluid also contains a pH control agentpresent in an amount sufficient to neutralize any acid or base presentin the solid material before the injection and any acid or basegenerated by the solid material during the injection.

In another embodiment, the viscosifier is a viscoelastic surfactant andthe particles include a material selected from waxes, polyesters,polycarbonates, polyacetals, polymelamines, polyvinyl chlorides,polyvinyl acetates, polyvinyl esters, polyacrylates, polymethacrylates,polycarboxylates, and mixtures of these materials.

Various other embodiments involve one or more of the following aspects:The fluid also contains a fluid loss additive that includes particlestoo large to pass into the formation pores. The solid additive particlesdo not form a substantially water impermeable film on formation orfracture faces. The particles are provided as one or more than one of asuspension, a dispersion, a latex, in the internal phase of anoil-in-water emulsion, inside structures selected from the groupconsisting of micelles, liposomes, vesicles, eucariote and procariotecells, and bacteria, or inside micelles.

In another embodiment, the particles include a member selected from longchain alcohols, aliphatic or aromatic solvents, fatty acids, fatty acidesters, mono, di or triglicerides of fatty acids, long chain alcoholphosphate esters, diacid esters, and mixtures of these materials.

Yet another embodiment is a fluid composition including a viscosifier,and a solid additive involving particles sufficiently small to entersubterranean formation pores; the additive involves a material that candegrade in the formation into a material that is soluble in a fluid inthe pores of the formation and is capable of breaking the viscosifier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the viscosity vs. time of a fluid that is viscosified witha viscoelastic surfactant system and contains different amounts of a waxinternal filter cake/matrix breaker of the Invention.

FIG. 2 shows the viscosity vs. time of a fluid viscosified with twodifferent concentrations of a viscoelastic surfactant system containingthe same amount of a wax internal filter cake/matrix breaker of theInvention.

FIG. 3 shows the viscosity vs. shear rate and time of a fluidviscosified with a viscoelastic surfactant system containing a waxinternal filter cake/matrix breaker of the Invention.

FIG. 4 shows the viscosity vs. shear rate and time of a fluidviscosified with a viscoelastic surfactant system and weighted with NaBrcontaining a wax internal filter cake/matrix breaker of the Invention.

FIG. 5 shows the viscosity vs. shear rate and time of a fluidviscosified with a viscoelastic surfactant system and weighted withCaBr₂ containing a wax internal filter cake/matrix breaker of theInvention.

FIG. 6 shows the viscosity vs. shear rate and time of a fluidviscosified with a viscoelastic surfactant system and weighted withCaCl₂ containing a wax internal filter cake/matrix breaker of theInvention.

FIG. 7 shows the effect of an internal filter cake former/matrix breakerof the Invention and its decomposition product as a function of pH onthe viscosity of a VES based fluid system as a function of shear rate.

FIG. 8 shows the effect of an internal filter cake former/matrix breakerof the Invention, with and without a pH control agent, on the viscosityof a VES based fluid system as a function of temperature.

FIG. 9 shows the viscosity vs. shear rate from the day of preparation tothe tenth day of storage of a betaine viscoelastic surfactant fluidsystem containing a dry emulsion internal filter cake former/matrixbreaker of the Invention.

FIG. 10 shows the viscosity vs. shear rate from the day of preparationto the tenth day of storage of a betaine viscoelastic surfactant fluidsystem containing a dry emulsion internal filter cake former/matrixbreaker of the Invention.

FIG. 11 shows the viscosity vs. shear rate from the day of preparationto the tenth day of storage of a betaine viscoelastic surfactant fluidsystem containing a dry emulsion internal filter cake former/matrixbreaker of the Invention.

FIG. 12 shows the viscosity vs. shear rate from the day of preparationto the tenth day of storage of a betaine viscoelastic surfactant fluidsystem containing a dry emulsion internal filter cake former/matrixbreaker of the Invention.

DETAILED DESCRIPTION OF THE INVENTION

Added fluid efficiency and ease of cleanup are the most importantproperties for any fluid loss or lost circulation additive used inoilfield treatment fluids. In treatments of subterranean formations, thetotal volume of fluids that needs to be pumped for completing thetreatment is strongly influenced by the quantity of fluid lost to thesurrounding matrix. (When comparatively little fluid leaks off, thefluid is said to be efficient; if comparatively large amounts of fluidleak off, the fluid is said to be inefficient. If 70% of the fluid leaksoff, then the fluid efficiency is only 30%.) The conventional approachrelies on building an external filter cake (for example on a fractureface or wellbore wall, external to the formation), sometimes with abreaker built into the system. (Also, for viscosified fluids used inoilfield treatments, it is important that there be a mechanism by whichthe viscosity can be reduced (that is, the fluid can be broken).Typically breakers are added to the fluid. Typically, the action of thebreaker is delayed or requires a trigger such as crushing ofencapsulated breakers, so that the fluid may perform its function beforethe break occurs. Proper placement is an important feature for anybreaker; it must be with the fluid or filter cake to be broken.) Once afluid loss or lost circulation additive invades the formation, mostconventional breakers (such as encapsulated oxidizing agents) orsubsequently added acid will be inefficient for cleaning it up;furthermore, often, the decomposed product (for example polymerfragments or crosslinked polymer fragments) will still be damaging tothe formation. Therefore, the formation of an internal filter cake (thatis, within the pores of the formation) is normally carefully avoided inorder to prevent formation damage.

In conventional fluids having polymers as the viscosifying agents,during the initial phase of the treatment the polymers are typicallyfiltered at the rock face to form a polymer filter cake thatsubsequently inhibits further losses. If, as is often the case, this isinadequate to limit fluid losses to the rock, additional components,usually solids called Lost Circulation Additives, or Fluid LossAdditives, or FLA's, are added to promote filter cake formation.VES-based fluids are polymer-free, which in itself is a major advantagesince polymers, remaining in the matrix once the treatment is over are amain source of formation damage, and consequently the fluid loss processis not governed by polymer filter-cake formation. FLA's may also beadded to VES-based fluids to reduce fluid loss, but this may be moredifficult because among other reasons there is no polymer to help.

To overcome the tendency of high fluid loss in polymeric and VES-basedfluids (in particular in hydraulic fracturing fluids and gravel packingfluids but in many other oilfield treatment fluids as well), variousfluid loss control additives have been proposed. Silica, mica, andcalcite, alone, in combination, or in combination with starch, are knownto reduce fluid loss in polymer-based fracturing fluids, by forming afilter cake which is relatively impermeable to water, on the formationface, as described in U.S. Pat. No. 5,948,733. Use of these fluid losscontrol additives alone in a VES-based fluid, however, has been observedto give only modest decreases in fluid loss as described in U.S. Pat.No. 5,929,002. The poor performance of these conventional fluid lossadditives is typically attributed to the period of high leak-off (spurt)before a filter cake is formed and to the formation of a filter cakepermeable to the VES-based fluid. Colloidal suspensions may also be usedto form FLA's. These are typically dispersions of discrete very smallparticles, spherical or elongated in shape, charged so that therepulsion between the same charged particles stabilizes the dispersion,such as those disclosed in U.S. Pat. No. 7,081,439 (Sullivan, et al.),incorporated herein by reference thereto. Disturbance of the chargebalance due for instance by removing the water, changing the pH oradding salt or water-miscible organic solvent, causes the colloidalparticles to aggregate resulting in the formation of a gel. Theparticles are less than 1 micron in size, and typically in the rangefrom about 10 to about 100 nanometers. Although one purpose of theInvention is to eliminate the need for previously known FLA's, it iswithin the scope of the Invention to use previously known FLA's with thecompositions and methods of the Invention.

The Invention will be described primarily in terms of hydraulicfracturing, gravel packing, acidizing, and fracture acidizing, althoughit is to be understood that the Invention may be used in many otherways, for example many other oilfield treatments. In hydraulicfracturing, most of the injected fracturing fluid contains a proppantsuch as sand or synthetic ceramic beads, so that when the pressure isreleased the proppant is trapped between the fracture faces and preventsthe fracture from completely closing, thus leaving a flowpath open. Theinjected fracturing fluid is normally viscosified. Increased viscosityresults in formation of a wider fracture, thus a larger flowpath. Aminimal viscosity is also required to transport adequate amounts ofproppant; the actual viscosity required depends primarily upon the fluidflow rate and the density of the proppant. In a typical fracturingprocess, such as hydraulic fracturing with aqueous fluids, the fractureis initiated by first pumping a high viscosity fluid with good tomoderate leak-off properties, and typically no proppant, into theformation. This initial fluid, typically referred to as a “pad”, isusually followed by a second fluid (fracturing fluid) of similarviscosity carrying an initially low concentration and then a graduallyor step-wise increasing concentration of proppant (in “stages”) into theextended fracture or fractures. The pad initiates and propagates thefracture but does not need to carry proppant. All the fluids tend to“leak-off” into the formation from the fracture being created orextended. Commonly, by the end of the job the entire volume of the padwill have leaked off into the formation. This leak-off is determined andcontrolled by the properties of the fluid (and additives it maycontain), the pumping rate and pressure, and the properties of the rock.A certain amount of leak-off greater than the minimal possible may bedesirable, for example a) if the intention is to place some fluid in therock to change the rock properties or to flow back into the fractureduring closure, or b) if the intention is deliberately to cause what iscalled a “tip screen-out”, or “TSO”, a condition in which the proppantforms a bridge at the some point in the fracture, stopping thelengthening of the fracture and resulting in a subsequent increase inthe fracture width. On the other hand, excessive leak-off is undesirablebecause it may waste valuable fluid and result in reduced efficiency ofthe job. In acid fracturing, the fracture fluid is an acid (or otherformation dissolving fluid such as a chelant-containing fluid) and thefluid normally does not contain proppant; the fracture is held open byasperities in the fracture faces caused by differential etching of theformation material. In matrix acidizing, an acid or other formationdissolving fluid is injected below fracture pressure and the fluidenters the formation and dissolves damaging materials and/or a portionof the formation. Proper leak-off control may be critical to the successof these and other oilfield treatments. Many solid particulateadditives, such as silica, mica and starch, have been developed tocontrol leak-off. Furthermore, in these and most other treatment types,after the treatment it is necessary to decrease the viscosity of thefluids, i.e. to break them.

We have found that certain solid degradable materials can be used toform internal filter cakes, that is filter cakes inside the formation,with the advantage that such filter cakes are not removed by fluid flowpast them, i.e. by erosion. These solid materials do not necessarilyform a water impermeable film on formation faces, such as fracturefaces. The degradation of the solid particles may occur naturally due tochemical or physical conditions, for example temperature or pH. Thisdegradation may optionally be accelerated or delayed if necessary. Thisdegradation may also optionally be triggered by contact with anotherfluid, such as another injected fluid, a formation fluid, or a producedfluid. Preferably, at least some of the degradation occurs beforeflowback or production. Injecting another fluid to promote thedegradation is not normally desirable because of potential costs andcomplexity, but is within the scope of the Invention. Furthermore, thedegradation products from some of the solid degradable particulatematerials of the Invention are breakers for polymeric viscosifiers, andthe degradation products of all of the degradable materials of theInvention are breakers for polymer-free (VES) fluid viscosifiers. Thebreaking effect occurs whether or not a filter cake is formed, forexample in a kill pill, or when the particle size is too small to formbridges, and thus to form a cake. We will call this material an“Internal Filter Cake/Matrix Breaker” or “IFCMB” with the understandingthat it has various uses in addition to one or both of internal filtercake former and matrix breaker (and with the understanding that undersome circumstances it may enter a formation and not form a filter cake).The IFCMB's are appropriately sized particles of materials that areinsoluble in the treatment fluid as pumped but degrade intowater-soluble or oil-soluble materials under bottom-hole conditionsafter being pumped.

We use the terms “solid” and “degrade” and “particle” here forsimplicity. It will be seen below that IFCMB's may be materials forwhich these terms are not strictly precise. For example, IFCMB's may bewaxes, cells, or intermediate to high molecular weight polymers that maybe “hard solids” or “soft solids”. The term “hard solid” is used here todesignate crystalline, amorphous or partially crystalline solids thathave softening temperatures (melting point, or glass transitiontemperature, whichever is higher) at least about 30 to 50° C. above roomtemperature). The term “soft solid” is used here to designatecrystalline, amorphous or partially crystalline solids that havesoftening temperatures close to room temperature (for example, less thanabout 30° C. above room temperature). IFCMB's may be “particles” in adifferent sense (for example they may be prepared in cells), and theymay become breakers by melting and/or by slow dissolution, or by releaseof smaller chemicals contained within them, without actual degradationof individual molecules or even of larger structures. The terms “solid”and “degrade” and “particle” are intended to include such additionalconnotations as will be apparent from the context.

Put simply, these materials, used in the proper way, greatly increasethe efficiency of many oilfield operations. These solid materials enterformation pores, optionally bridge there to form a filter cake, and thendegrade to break the fluid's viscosifier. They offer high fluidefficiency and complete breaking of the fluid; they minimize the amountof fluid that leaks off, break any fluid that does leak off and inaddition any fluid that does not leak off, and they break any filtercake that might form.

These IFCMB's have a significant impact on treatment fluid efficiency byinvading a formation and then degrading under bottom-hole conditionsinto water-soluble or oil-soluble low molecular weight materials, henceeliminating formation damage. The Invention in particular provides foreffective treatment of high permeability formations because it reducesthe fluid volumes needed. The Invention is particularly suited for usewith polymer free fluids that do not form filtercakes. The Invention isespecially useful in gravel packing and the like, where near-wellboredamage is often a particularly serious problem. The Invention makes itpossible to treat wells previously eliminated as candidates due to thelow fluid efficiency that would have been expected. IFCMB's may alsofunction as a self-destructing diverting agent. They may also be used inkill pills, which can be difficult to break because mechanisms oftenavailable for breaking (such as crushing of encapsulated materials, orlater addition of another component) cannot be used with kill pills.Furthermore, if kill pill leak off is a problem, they prevent it.

In treatments that typically include multiple stages, such as mosthydraulic fracturing, acid fracturing, frac-packing, and gravel packingembodiments, the IFCMB may be added in the pad, throughout the treatmentor to only some of the stages, such as some of the proppant, gravel,acid, or diversion stages. A self-destructing diverter, fluid lossadditive, filter cake former, and breaker is particularly useful inhydraulic fracturing, frac-packing, and gravel packing becausemechanical removal methods are impossible and methods involvingcontacting the additive with an additional fluid are not alwayspractical. Since the additive is self-destructive, it can be placedinside the matrix (internal) without detracting from its effectiveness.Since the additive is self-destructive, it can be used in a kill pillthat would otherwise be difficult to break. The compositions and methodsof the Invention are also particularly useful in cases where it isdesirable to limit leak off but to allow a certain amount of treatmentfluid to enter the formation, for example for the purpose of alteringformation wettability or oil or water saturation.

Treatment fluids used with the compositions and methods of the Inventiontypically also contain other materials such as demulsifiers, corrosioninhibitors, friction reducers, clay stabilizers, scale inhibitors,biocides, breaker aids, mutual solvents, surfactants, anti-foam agents,defoamers, viscosity stabilizers, iron control agents, diverters,emulsifiers, foamers, oxygen scavengers, pH control agents, buffers, andthe like. Compatibility of IFCMB's with such additives should be checkedin the laboratory. The treatments of the Invention are conductednormally, taking into account improved fluid efficiency and breaking.The treatment fluid and additives are transported to the site, mixed,stored, and pumped in the usual ways for the respective chemicals.

The Invention is carried out by considering information about the well,the formation, the fluids and additives available, and criteria for asuccessful treatment, and preparing an optimized plan for maximizingtreatment performance according to the data and the criteria. This isusually done by analyzing the well using treatment design and evaluationsoftware; for example, in hydraulic fracturing software, pressuregradients are combined with fracture length and height evolutionalgorithms, complete leak-off information, and the effects of multiplefluid injections and their temperature changes.

The optimal size, or size distribution, and concentration of theadditive for fluid loss control can be determined by choosing thedesired leak-off parameters and measuring leak-off with samples of theintended fluids and of the formation or of a rock similar to theformation. Leak-off is defined by the “spurt”, which is the initialrapid leak-off of fluid before a filter cake barrier is formed, and bythe subsequent leak-off that occurs even after a filter cake is formedand is governed by the viscosity and the wall-building propensity of thefluid and its components. The term “wall-building” is used becauseexternal filter cakes are usually formed on a “wall” such as a fractureface, but it is used here to refer to a conventional external filtercake, an internally formed filter cake, or both. Leak-off is typicallymeasured in units of volume or mass per unit area per unit time; for afixed geometry in the laboratory, it may be measured simply by mass orvolume as a function of time. Measurement of leak-off, prediction, andcontrol of leak-off by formation of a filter cake with a fluid lossadditive, are familiar to those of ordinary skill in the arts of wellstimulation and sand control.

In a typical propped hydraulic fracturing treatment, the pad fluid isdesigned to completely leak off into the formation; this completeleak-off ideally coincides precisely with the termination of pumping ofthe proppant stages (pumping a slurry containing proppant) so that thenewly created fracture is propped open. In most, if not all, fracturetreatments, leak-off in excess of the minimal amount required foroptimal fluid efficiency and fracture dimensions is inevitable andundesirable. Thus the volume of the pad, relative to the volume of thefracture fluid, in a job design is determined by a large number offactors governed by the properties of the fluids and of the formationand the desired final fracture dimensions. The selection of a suitablepad volume is within the ability of those skilled in the art, takinginto account the affects of the additive of the Invention on fluidefficiency.

Because in some embodiments the pad and the fracture fluid arepolymer-free and the fluid loss additive and breaker are fullydegradable, neither the near-wellbore formation nor the proppant bedleft in the fracture after the job contains deleterious polymers orsolids, as would be the case if the fracture fluid contained any polymeror if the fluid loss additive and breaker were not fully degradable.Therefore fracture conductivity is high and skin is low. However, someof the degradable materials suitable for use as IFCMB's of the Inventionrelease acid when they degrade; acid breaks some polymer and crosslinkedpolymer viscosified fluids (such as polysaccharides such as guars andsubstituted guars) and so some IFCMB's may be used with somepolymer-based fluids. Those skilled in the art know which polymers arebroken by acid and under what conditions.

The methods of the present Invention may still work satisfactorily informations in which some of the pore throats at the fracture or wellborefaces are too small for some of the additive particles to enter theformation. In that case, external filter cakes in addition to internalfilter cakes form, and then degrade. In low permeability formations,only minimal fluid loss control is generally needed, and provision ofviscosity to widen the fracture is a relatively more important role ofthe pad fluid. We define low permeability formations here as havingpermeabilities of less than about 20 mD, especially less than about 10mD, and most especially less than about 2 mD. In high permeabilityformations, fluid loss control is more difficult and more critical andadditives more easily penetrate the formation.

While this Invention is not bound by any particular mechanism ofoperation or theory, it appears that one can define three permeabilityranges for a given IFCMB additive particle size distribution and a givenVES fluid:

First, a higher permeability range, in which, and above which, theadditive particles and VES fluid micelles can enter the formationwithout forming a filtercake; in this case leak-off is high and/orpenetration is deep.

Second, an intermediate permeability range, depending upon thedimensions of the additive and of the initially present VES fluidmicelles, and the stability of the micelles of the VES fluid; theadditive enters the formation and forms an internal filter cake, and themicelles either enter, or they break and then enter as surfactantmolecules or smaller micelles that then reform micelles of the originalsize.

Third, a lower permeability range below which, still depending upon thedimensions of the additive and of the initially present VES fluidmicelles, and the stability of the micelles of the VES fluid, theadditive does not enter, the micelles either enter, or they break andthen enter as surfactant micelles that then either reform or do notreform micelles. (Additive that does not enter forms an externalfiltercake; VES may stop entering the formation owing to the filtercake.)

Although there is not universal agreement on the precise relationship ofparticle size, pore dimension, and bridging, we will use the followingguidelines here. Particles having diameters greater than about one-third(some researchers say up to one half) of a pore throat diameter areexpected to bridge at or near the formation face. Particles smaller thanthat but larger than about one-seventh of a pore throat diameter areexpected to enter the formation and be trapped and form an internalfilter cake. Particles smaller than about one-seventh of a pore throatdiameter are expected to pass into the formation and through the poresof the formation without substantially affecting flow. It is to beunderstood that there are other important factors such as distributionsof particle size and of pore size, flow rate, particle concentration,and particle shape.

The concentration range of IFCMB's may be for example from about 0.01%to about 10% by weight of the fluid, for example from about 0.1 to about3% by weight of the fluid. The amount of IFCMB is determined in the sameway as the amounts of breakers and fluid loss additives are determined.

It should be understood that throughout this specification, when we listor describe a concentration or amount range as being useful, orsuitable, or the like, we intend that any and every concentration withinthe range, including the end points, is to be considered as having beenstated. Furthermore, each numerical value should be read once asmodified by the term “about” (unless already expressly so modified) andthen read again as not so modified unless otherwise stated in context.For example, “a range of from 1 to 10” is to be read as indicating eachand every possible number along the continuum between about 1 and about10. In other words, when we express a certain range, even if weexplicitly identify or refer to only a few specific data points withinthe range, or even to no data points within the range, it is to beunderstood that the inventors appreciate and understand that any and alldata points within the range are to be considered to have beenspecified, and that the inventors have possession of the entire rangeand all points within the range. Measurement of breaking and ofreduction of fluid loss, and prediction and control of breaking and ofreduction of fluid loss, are familiar to those of ordinary skill in thearts of well stimulation and sand control.

One skilled in the art and knowing the properties of a formation and theproperties of fluid components could determine when an IFCMB and/or aVES fluid might extensively enter (leak off into) a formation andwhether or not an external or internal filter cake would form. Thiswould depend, for example, upon the nature of the VES, the sizedistribution of the IFCMB additive, and the formation permeability andpore structure. U.S. Pat. No. 5,964,295 (Brown, et. al., issued Oct. 12,1999; hereby incorporated by reference in its entirety) describesseveral types of effective VES fluids, although the Invention is notlimited to those VES fluids, and explains the inter-relationships ofviscosity, and fluid loss and leak-off in relatively low permeabilityformations (less than about 10 mD for the VES fluid surfactantsdescribed in U.S. Pat. No. 5,964,295, but different for different VESfluid surfactants).

Any non-polymeric fluid, for example VES based fluid, can be used in theInvention that is compatible with the formation, the formation fluids,and the other components of the fluid, including IFCMB additives.Particularly effective non-limiting examples of fluids are thosedescribed in U.S. Pat. Nos. 5,551,516; 5,964,295; 5,979,555; 5,979,557;6,140,277; and 6,258,859, all hereby incorporated by reference.Vesicle-based fluids may be used, such as those described in U.S. Pat.No. 6,509,301, hereby incorporated by reference. Any polymer-based orcrosslinked polymer-based fluid may be used, provided that the polymeris broken by acid, and that the IFCMB selected releases acid ondegradation.

In some cases, a certain amount of leak-off is desired, for example sothat a tip screen-out occurs in fracturing, a condition in which theproppant forms a bridge, preferably at the end of the fracture away fromthe wellbore, stopping the lengthening of the fracture and resulting ina subsequent increase in the fracture width. For example, hydraulicfracturing followed by gravel-packing in a single operation, sometimescalled a frac-pac, fracpac, frac pac, frac and pac, or STIMPAC™,sometimes with a deliberate tip screen-out to generate a short widefracture, is usually performed in relatively high permeabilityformations for sand-control purposes. However, such operations aresometimes performed in low permeability formations, occasionally forsand control, but also for other reasons, for example to bypasspermeability damage near the wellbore caused by scaling or to improveupon poor communication between the wellbore and the formation or aprevious fracture, or in formations in which perforating createsdamaging fines, or for other reasons. Such jobs designed to generateshort wide fractures may also be performed without subsequentgravel-packing when sand control is not an issue. The methods of thepresent Invention can be used in any of these cases (fracturing followedby gravel packing and/or fracturing for short wide fractures, in eithercase with or without deliberate tip screen-out). The particle sizedistribution of the IFCMB is adjusted to provide a suitable leak-offrate.

The acid used in the matrix acidizing and acid fracturing methods ofthis Invention can be any acid used in acid fracturing, includinggelled, self-diverting, and delayed acids. Commonly used, but notlimiting, acids are hydrochloric, hydrofluoric, fluoboric, acetic, andformic acids and mixtures thereof and those acids in the form of oilexternal emulsions (for reaction rate retardation), or oil internalemulsions (for hydrocarbon solvency). The acids can contain additivessuch as corrosion inhibitors and chelants used to help dissolve rockcomponents and keep them in solution. Gelled, self-diverting, anddelayed acids can be gelled with suitable VES's.

Although in conventional propped fracturing the most common way tocontrol fluid loss is to build an impermeable or reduced-permeabilityfiltercake on the fracture walls (faces), in acid fracturing, especiallywith a low viscosity ungelled acid, pad viscosity is important for fluidloss control. In fact, if the pad of a fracturing treatment is anuncrosslinked polymer fluid, some leak-off control will still resulteven if no or little filtercake is formed, because as the pad leaks offinto the formation it will form a layer of viscous fluid in the firstseveral inches of the formation adjacent to the fracture. Also, if theVES has higher low-shear viscosity than high-shear viscosity, which iscommon, then as the VES leaks off a short distance into the formation,the flow rate decreases, the shear rate therefore decreases, and thefluid becomes more viscous. Such effects can reduce low viscosityungelled acid leak-off better than a wallbuilding system that dissolvesor decomposes in acid. In these cases, an IFCMB would be particularlysuitable in the pad because it may be selected to allow penetration ofthe fluid a specified distance into the formation and then limit furtherpenetration. This allows acid treatment a certain selected depth intothe formation before a filter cake forms which performs the verydesirable function of diverting subsequent acid.

Sometimes acid fracturing is performed with a series of alternating pad,acid, pad, acid, etc. stages in order to optimize coverage. The firstnon-acidic pad initiates a fracture for the first acid stage to follow.That first acid stage etches a portion of the fracture face. Subsequentstages of pad and acid repeat the process until the designed treatmentvolumes have been injected and the desired fracture has been created. Inthe past, this process has always used a gelled pad, such as crosslinkedpolymer, or uncrosslinked polymer at high concentrations such as 4.8 to7.2 g/L (40 to 60 lbm/1000 gallons or ppt). The method of the Inventioncan be used in at least the first pad and sometimes in all the padstages. If wormholes are formed, or very fine natural fractures areencountered, that attract acid and minimize the amount of acid thattravels along the faces of the growing hydraulic fracture (which iscommonly the case in the types of limestone and dolomite formationssubjected to acid fracturing or fracturing with alternating pad and acidstages), some or all pad and/or acid stages may beneficially containIFCMB components to limit acid penetration to a desired depth into theformation and to divert acid to other locations in a fracture orformation. Similarly, matrix acidizing may be performed with alternatingstages of acid and another fluid, such as a diverter, some or all ofwhich may be viscosified; the IFCMB's of the Invention may be includedin some or all of either the acid or the other fluid to control flowand/or to break a viscosifier.

An important advantage to the use of the IFCMB's of the Invention influid loss additives, lost circulation control additives, in externaland internal filter cakes, and even in treatments in which no filtercakes are formed (such as in kill pills), is that the degradationproducts generated in the self-destruction process function as a breakerfor viscoelastic surfactant viscosifying agents. Certain acids (such ascarboxylic acids), alkyl sulfates, phenols, amines, carbon dioxide,alcohols and other materials are known to damage or destroy either themicelle/vesicle structures formed by some viscoelastic surfactants usedto viscosify drilling, completion and stimulation fluids or, in somecases, to destroy the surfactants themselves. Whether a givendegradation product (for example an alcohol, an acid, the anion of anacid, etc.) will be a breaker for a given VES depends upon thestructures, charges, hydrophilic/lipophilic nature, etc. of thedegradation product and the VES. Examples of such structure breakers andsurfactant breakers, and others, are described in U.S. Pat. Nos.6,881,709, and 6,908,881, both assigned to the assignee of the presentapplication and hereby incorporated in their entirety. Sometimes thesame material can be a breaker for some surfactants, or at someconditions, and have no effect, or even be a stabilizer, for the sameand/or other surfactants and/or at other conditions. Whether adegradation product will be a breaker, and under what conditions, canreadily be determined by simple experiments well known to those in theart.

The IFCMB's of the Invention may also be used in conjunction withconventional breakers, that may be used to provide additional breakingcapacity where they can be used, such as in fractures. This may beparticularly beneficial in cases where an external filter cake must bebroken. The IFCMB's of the Invention may also be used in conjunctionwith internal breakers designed to work in the pores of a formation.Examples are given in copending application “Internal Breaker forOilfield Fluids,” inventors Philip Sullivan, Yiyan Chen, Belgin Baser,Carlos Abad, Mehmet Parlar, and Gregory Kubala, filed Sep. 18, 2006.Breaker aids such as activators, delay agents or stabilizers may also beused specifically in conjunction with the IFCMB's of the Invention.

There are several classes of materials, for example polymers, that havethe right chemistry and can be made in suitable sized particles so thatthey form internal filter cakes and then degrade under bottom-holeconditions to form breakers for VES fluid systems. Materials (such aswaxes) that melt as formations warm up after fluids are injected mayalso be used; in that case, the “degradation” products are simplyhydrocarbons. Degradation is, for example, triggered by a temperatureincrease or by a chemical trigger such as a change in pH. Degradationmay also be sufficiently slow, even at formation conditions, that theparticles do not degrade until after all the fluid has been pumped.(That would, for example, be the case for certain esters at certainconditions (for example of temperature and pH)). In general, IFCMB'sgenerate effective breakers if the degradation products are, forexample, polyelectrolytes, acids, oils or polymers having hydrophobicportions.

Many of the polymers and waxes described here are commonly available inparticle size ranges of tens of nanometers to hundreds of microns.Suspensions of such particles are sometimes incorrectly called“emulsions” if they are hydrocarbon particles stabilized in aqueousfluids (by analogy to true “oil in water” emulsions of two liquidphases). Materials available as such suspensions or dispersions aregenerally available in smaller sizes, down to microns; they may beobtained “dry” or in the usual suspension or dispersion form. If sizesof any of the materials are needed that are not commercially available,they can be made by known methods. Some of the polymers are made in bulkand then fabricated into the desired size and shape, while most of thepolymers are polymerized directly in the desired size and morphology,for example via suspension (for particles in the micron size) oremulsion (for particles in the submicron size) polymerization.

Examples of materials suitable for IFCMB's include:

(1) Polyesters

In general Structures 1 and 2 are typical of the structures of thesematerials; they release carboxylic acids and alcohols upon degradation.Degraded products are usually environmentally friendly.

In Structure 1, R is typically a linear short chain aliphatic, saturatedor unsaturated, hydrocarbon ranging from zero to about 10 carbon atoms.These are made by esterification, with a suitable alcohol, of adicarboxylic acid such as oxalic acid, propanedioic acid, maleic acid,fumaric acid, glutaric acid, adipic acid, dodecyldioic acid, or anaromatic diacid such as 1,2-phthalic acid, 1,3-phthalic acid or1,4-phthalic acid (also commonly known as isophthalic acid, terphthalicacid, etc.) and their derivatives, for example alkyl substitutedphthalic acid derivatives, or chlorinated or fluorinated phthalic acid.They are also made by esterification with a suitable alcohol of ananhydride such as maleic anhydride, glutaric anhydride, adipicanhydride, phthalic anhydride; or by esterification with a suitablealcohol of a diacyl halide (e.g. diacyl chloride) such as for exampleglutaric acid dichloride, adipoyl chloride, phthaloyl chloride. R mayalso include a branched structure such as in 2-methyl dipropionic acid.R′ is typically a linear short chain aliphatic or aromatic chain havingfrom about 2 to about 12 carbon atoms, such as in, for example, ethyleneglycol, butylene glycol, hexanediol; a diphenol such as resorcinol;hydroquinone; or a branched structure such as propyleneglycol. Alsosuitable as precursors for Structure 1 are polyols such as glycerol,sorbitol, manitol; monosaccharides such as glucose, fructose, galactose,manose; disaccharides such as maltose, or sucrose; or polymeric diolssuch as a polyether such as polyethyleneglycol, polypropyleneglycolpolyphenylene oxide, polyphenoxyphenylmethane, polyphenoxyphenylpropane;polymeric polyols such as phenol formaldehyde copolymers (resols, ornovolacs); cresol formaldehyde copolymers; epoxy resins; alkyl resins;etc. In Structure 1, n is typically from about 1 to about 100,000, andmore typically from about 10 to about 10,000.

In Structure 2, suitable polyesters, are, for example, selected fromlactide, glycolide, polylactic acid, polyglycolic acid, copolymers ofpolylactic acid and polyglycolic acid, copolymers of glycolic acid withother hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containingmoieties, copolymers of lactic acid with other hydroxy-, carboxylicacid-, or hydroxycarboxylic acid-containing moieties, polymers of3-hydroxypropionic acid, epsilon caprolactone, 4-hydroxybutyric acid,5-hydroxypentoic acid, 2-hydroxy valeric acid, 3-hydroxy valeric acid,and mixtures of those materials. Preferred poly(alpha-hydroxy acids) arepoly(glycolic acid), poly(DL-lactic acid) and poly(L-lactic acid).Polylactones such as poly(epsilon-caprolactone),poly(delta-caprolactone), poly(delta-valerolactone) andpoly(gamma-butyrolactone are suitable. Other suitable polyesters includethose polymers of hydroxyacetic acid (glycolic acid) with itself orother hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containingmoieties described in U.S. Pat. Nos. 4,848,467; 4,957,165; and4,986,355, all three hereby incorporated by reference. Suitablepolyesters are also described in U.S. Patent Application PublicationNos. 2003/002195 and 2004/0152601, both of which are hereby incorporatedby reference and are assigned to the assignee of the presentapplication. Particularly suitable polyesters, that are known to bebiodegradable, are polylactic acid (polylactide) and polyglycolic acid(polyglycolide), shown as structures 3 and 4. The use of such materialsas external filter cake formers and VES breakers, with or without a pHbuffer that allows the VES fluid systems to maintain their viscosity ifthere is some free acid in the polymer or while some of the polymerhydrolyzes, was described in U.S. Patent Application Publication No.2005-0252659. The use of certain esters (orthoesters such as trimethylorthoacetate) as delayed breakers for VES's was reported in U.S. PatentApplication No. 2005-0045328, but these were not polymers and were notused as solid particles. However, use of solid polyesters withoutexternal filter cake formation, e.g. as internal breakers, or asbreakers of fluids such as kill pills, or as internal filter cakeformers, is not known. Suitable buffers include, but are not limited to,sodium, potassium and ammonium sesquicarbonates, carbonates, andbicarbonates. Soluble oxides, including slowly soluble oxides such asMgO, may also be used. The choice of buffer depends in part upon the VESsystem used. The appropriate buffer concentration depends upon the IFCMBconcentration and primarily upon the desired delay before the onset ofthe break. Since polyesters degrade into organic acids, they may also beused as IFCMB's with fluids viscosified with suitable linear orcrosslinked polymers (such as many polysaccharides, such as guars andderivatized guars).

(2) Polycarbonate Copolymers and Polycarbonates

Typical generic structures are shown in Structures 5 and 6. ForStructure 5, R and R′ are typically a residue of a diol, such as analiphatic or aromatic chain derived from structures, for exampleethylene glycol, butylene glycol, hexanediol; a diphenol, for exampleresorcinol; a hydroquinone; or more typically a condensation product ofphenol and formaldehyde or acetone, for example bisphenol F (bisphenolformaldehyde) or bisphenol A (bisphenol acetone). Branched orsubstituted variations may also be used, for example alkylated bisphenolA, or propyleneglycol. In Structure 5, n is typically from about 2 toabout 100,000, for example from about 10 to about 10,000. In Structure6, R is typically an aliphatic or aromatic chain derived from structuressuch as ethylene glycol, butylene glycol, hexanediol, etc.; diphenolssuch as resorcinol; hydroquinones; or more typically a condensationproduct of phenol and a ketone or aldehyde, for example formaldehyde, orand acetone, for example bisphenol F (bisphenol formaldehyde) orbisphenol A (bisphenol acetone). Branched or substituted variations mayalso be used, for example alkylated bisphenol A, or propyleneglycol. InStructure 6, n is typically from about 2 to about 100,000, for examplefrom about 10 to about 10,000. These types of materials release carbondioxide and alcohols upon degradation. The CO₂ pressure thus producedwould further energize the system and facilitate the clean up process.Furthermore, the carbon dioxide would make the environment acidic, sopolycarbonates may also be used as IFCMB's with fluids viscosified withsome linear or crosslinked polymers that are broken by acids, as well aswith VES's that are broken by alcohols or acid.

(3) Polvacetals

A suitable degradable class of polymer is polyacetals (Structure 7), inwhich R1 and R2 are either alkyl or H groups. A particular example ispolyformaldehyde (Structure 8) or polyoxymethylene such as DELRIN™ (theDuPont Company) which gives formaldehyde upon decomposition, or itscopolymers with for instance tetrahydrofuran to yield copolymers such asCELCON™ (Celanese Plastics Co.), or polyvinyl butyral. Polyacetals havelow degradation temperatures and thus are good candidates as IFCMB'sthat undergo thermal degradation readily. Typically, n is from about 2to about 100,000, and more typically from about 2 to about 10,000 Thelow molecular weight polyformaldehyde obtained by degradation ofpolyoxymethylene or the polyformaldehyde encapped with alkyl structuresresulting form the degradation of the above listed copolymers may alsobe breakers for the surfactant.

(4) Melamine polymers and melamine polymer derivatives

This type of polymer (for example melamine-formaldehyde resin (structure9) in which n is typically from about 2 to about 5,000 can easily bederivatized due to the presence of active functional groups. Therefore,degradability can be fine-tuned, and these materials may be moreflexible than other polymers. Degradation products having differentmolecular weights may be used as VES breakers. Melamine monomer oroligomers may be released through the hydrolysis of these polymers. Themonomeric or oligomeric species interfere with the VES surfactantstructure and disrupt the alignment of the surfactant molecules, thusreducing the fluid viscosity.

Similarly urea formaldehyde polymer derivatives can also be used, astheir thermal stability is lower than that of melamine formaldehyderesins, are their decomposition products can also act as breakers frothe surfactant fluid.

(5) Polyvinyl materials such as Polyvinyl chlorides, Polyvinyl acetates,Polyvinyl esters, Polyacrylates, Polymethacrylates, andPolycarboxylates.

Polyvinyl chloride (shown in Structure 10, n is typically from about 10to about 100,000) is an easily degradable polymer. The chloride atoms onthe polymer backbone makes it very susceptible to chemical reaction,releasing HCl. Polyvinyl acetate (Structure 11, n is typically fromabout 10 to about 100,000) can also be hydrolyzed into polyvinyl alcoholthat degrades into small molecules. Degradation of polyvinyl acetatesreleases acetic acid, so they may also be used as IFCMB's with fluidsviscosified with suitable linear or crosslinked polymers (such as manypolysaccharides, such as guars and derivatized guars).

Polyvinyl esters, for example those obtained by copolymerization withother carboxylic acids such as butyric, propionic, oleic, dodecanoic orneodecanoic, (also known as versatic acid) may also degrade to releaseacids of various hydrocarbon lengths, that may act as breakers for VESfluids. Polyacrylate polymers and copolymers and polymethacrylatepolymers and copolymers, for example methyl acrylate, methylmethacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butylmethacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, laurylacrylate, lauryl methacrylate, oleyl acrylate, oleyl methacrylate,stearyl acrylate, stearyl methacrylate, and others may also degrade,releasing alcohols of various hydrocarbon lengths, that may act asbreakers for VES fluids. The hydrolyzed backbone polymer of apolycarboxylate may also act as a breaker for VES fluids.Polycarboxylates obtained by polymerization or copolymerization ofacrylic acid, or methacrylic acid, or obtained by hydrolysis ofpolyacrylamide or of polymethacrylamide polymers or copolymers may alsobe used as breakers for VES fluids.

(6) Waxes

Small wax particles can be used as an IFCMB material if used when theformation in which the filter cake is formed has been cooled to belowthe melting point of the wax; these are obtained as aqueous suspensionsor dispersions of fine particles, usually less than 1 micron. Theparticles are solid at the temperature of the injected fluid, and thatfluid cools the formation sufficiently that the particles enter theformation and remain solid. Early in the treatment, if any wax meltsupon contact with a portion of the formation that is above the waxmelting point, then incoming injected fluid solidifies this wax, furtherblocking the pores. After the treatment, when the formation heats backup to its original temperature, the wax melts and flow resumes. The waxmolecules are VES breakers. Aqueous wax suspensions or dispersions arecommonly used in wood coatings; engineered wood processing; paper andpaperboard converting; protective architectural and industrial coatings;paper coatings; rubber and plastics; inks; textiles; ceramics; andothers. They are made by such companies as Hercules Incorporated,Wilmington, DE, U.S. A., under the trade name PARACOL®, Michelman,Cincinnati, Ohio, U.S.A., under the trade name MICHEM®, and ChemCor,Chester, N.Y., U.S.A. Particularly suitable waxes include those commonlyused in commercial car washes. In addition to paraffin waxes, otherwaxes, such as polyethylenes and polypropylenes, may also be used.However, it must be kept in mind that if the molecular weight ofmaterials such as polyethylenes and polypropylenes is too high, theywill not participate in the micellar phase and thus will not be suitablefor use in the Invention. Emulsification in tiny droplets will helpalleviate this problem. Furthermore, particle size and the suspensionpackage required to maintain the wax dispersed in the fluid play animportant role in the ability of these particles to act as effectivebreakers, and so such materials must be tested with specific VES fluids(and their additives) under application conditions to ensuresuitability.

(7) Polyvinyl alcohols

Copending U.S. patent application Ser. No. 11/316,021, filed Dec. 21,2005, describes the use of certain polyols, such as polyvinyl alcohols,as IFCMB's.

(8) “Encapsulated” IFCMB's

IFCMB's may be encapsulated particles, for example (a) by placing theminside structures such as micelles, liposomes, vesicles, eucariote andprocariote cells, or bacteria, or (b) by incorporating them in theinternal, non-aqueous, phase of emulsions, including micro-emulsions,and including emulsions that could be used to form dry emulsions, or c)by using them as the solid portion of suspensions or dispersions such aslatexes (a term often used for stabilized suspensions of solid polymersin water), including suspension of so-called “dry emulsions” which areactually dry fine powders as received and handled until they aresuspended. Recall that suspensions or dispersions of some polymer andwax particles are sometimes called “emulsions” if they arehydrocarbonaceous particles stabilized in aqueous fluids (by analogy totrue “oil in water” emulsions of two liquid phases). These new materialsshould be distinguished from conventional encapsulated breakers, inwhich solid breakers are enclosed in polymeric shells, that aretypically the size of proppants, and the breakers are released bydissolving the capsules, by crushing or rupturing the capsules, or bydiffusion. “Encapsulated” IFCMB's (quotation marks will not be usedfurther below) may be liquids as well as solids. The manufacture and useof suspensions or dispersions (including dry emulsions) are very wellknown in the paint, adhesive, paper, textile, and other industries.Latexes are produced by various mechanisms such as emulsionpolymerization, mechanical emulsification, ultrasonic emulsification,flow through predefined sized porous membranes, and others. Latexes areoften used in the pharmaceutical or agricultural industries to releasechemicals such as drugs or insecticides slowly.

Note, in this specification, the term “emulsion” is used for a systemthat has more than one liquid phase; the terms “dispersion” andsuspension are used for a system in which solid particles are suspendedin a liquid, and the term “dry emulsion” is used for dry solid particlessuch as those found suspended in a suspension or dispersion. The term“latex” is conventionally used for a stabilized suspension of smallsolid polymer particles and will be so used here; “latex” or “latexparticles” are sometimes also used here to designate those particlesthemselves. In a latex, the polymers useful in embodiments of theInvention are dispersed and stabilized in an aqueous medium. Anysuitable means of stabilizing and dispersing the polymer in an aqueousmedium may by used. The actual technique used is not particularlyimportant as long as the polymer, or polymer particle, remains dispersedin the aqueous medium at least for the time period necessary fordelivery to the formation.

It should be noted that the structure, emulsion, or suspension and theIFCMB it contains should be compatible with conventional additives,including, but not limited to clay control additives, iron controladditives, temperature control additives, and the like. As alreadynoted, the solid may be a polymer such as a latex resin (also termedlatex polymer) stabilized in an aqueous medium. As used herein, theterms “latex resin”, “latex”, or “latex polymer” refers to a dispersionof a polymer which may be prepared by polymerization techniques such as,for example, by emulsion polymerization, and further, includes polymersprepared by these techniques where the average diameter of the dispersedpolymer ranges from the nano scale, such as nanolatexes, to microgelswhich are on the order of from about 10⁻³ microns to about 10³ micronsin diameter, including any functional diameter therebetween. The latexpolymer may be an aqueous suspensions or dispersion of finely dividedpolymer particles. Any practical blend of latex size may be used inaccordance with the Invention, such as a blend of a latex polymer with ananolatex polymer.

Latex resins are prepared synthetically by polymerizing monomers thathave been emulsified with surfactants. For example, in preparing a latexby polymerization, first, typically, sufficient surfactant is dissolvedin an aqueous solvent (water, or in some cases mixtures of water andsmall amounts of co-solvent) to form spherical micelles (therefore atconcentrations above the critical micelle concentration). The interiorof the micelle provides the site necessary for polymerization. A monomer(for example styrene, hydroxyl ethyl acrylate, methyl methacrylate,vinyl acetate, vinyl versatate, acrylic acid, oleyl acrylate, iso-octylacrylate, or butyl acrylate) and a water-soluble free radical initiatorare added and the whole batch is mixed. Batch, semibatch, and continuousprocesses are all commonly used to produce polymers by emulsionpolymerization. The water rapidly quenches anionic and cationic chainends. Core-shell latexes are also useful in some embodiments of theInvention. Readily known to those of skill in the art, preparation ofcore-shell latexes is commonly performed by two-stage emulsionpolymerization. In the first stage a polymer is formed as the core byemulsion polymerization. In a second stage, polymerization of a shellcrosslinked with the core is prepared by emulsion polymerization. Theterms “crosslinked” and “crosslinking” as used herein refer to chemicalbonds between existing polymer chains. Typically latex resins (polymers)are stabilized in the aqueous environment by surfactants, by mixtures ofsurfactants, and by mixtures of surfactants and hydrocolloids, by thefunctionality of the shell in a core-shell latex, or by a combination ofsurfactant and the functionality of the shell. Nonlimiting examples oflatex types, which may be useful, include latexes of acrylic copolymers,vinyl monomers such as vinyl chloride or fluoride or vinylidene chlorideand fluoride, polyvinyl acetate, which by hydrolysis can yield latexescontaining polyvinyl alcohol, crosslinked polyvinyl alcohol/vinylacetate copolymers, polyvinyl pyrrolidone, styrene-butadiene latexes,and the like. Any suitable latex may be used according to the Invention.Additional examples include vinyl acetate homopolymers, and copolymersof vinyl acetate and/or one or more of various acetates such as vinylversatate (VEOVA), maleic acid or esters, acrylates and methacrylates,or other esters; and copolymers of acrylic acid and/or one or more ofvarious acrylates such as methacrylic acid, methyl acrylate, methylmethacrylate, ethyl acrylate, n-butyl acrylate, and 2-ethyl hexylacrylate. The most preferred latexes useful for the Invention contain alabile bond, for example an ester bond, in the form of an acrylate,methacrylate or vinyl ester component.

A wide variety of breakers and breaker precursors may be encapsulated toform IFCMB's; not every breaker or breaker precursor can be encapsulatedby every encapsulation method, but those familiar with a givenencapsulation method will know what can be encapsulated. Representativebreakers suitable for these types of encapsulation include long chainalcohols, linear or branched, saturated or unsaturated such asdodecanol, tridecanol, tetradecanol, pentadecanol, hexadecanol, orproducers of these alcohols; acids or producers of acids such as stearicacid, oleic acid, dodecanoic acid, linoleic acid, erucidic acid, etc.;hydrocarbon solvents such as tetradecane, hexadecane, octadecane orpropyleneglycol monomethyl ether, ethyleneglycol monomethyl ester, etc;crude or mineral oils; waxes; oligomeric polymers, copolymers, andamphiphilic copolymers, such as polystyrene sulfonate copolymers,polyvinyl alcohol copolymers, partially hydrolyzed polyvinyl esters,polynaphthalene sulfonate copolymers, phenolic resins, polyacrylates,polylactic acid, polyglycolic acid, polyhydroxyalkanoates; short chainsurfactants; enzymes or proteins; oxidizers, and chelants. Many breakersand breaker precursors are described in U.S. Pat. Nos. 6,881,709 and6,908,888, each of which is assigned to the assignee of this applicationand each of which is hereby incorporated in its entirety. Examples ofactivators that can be encapsulated include acids and bases.

A particularly suitable method of encapsulation of liquids andlow-melting solids is formation of dry emulsions, for example accordingto the methods described in U.S. Pat. Nos. 6,528,473, 6,596,337, and6,537,583 and U.S. Patent Application Publication No. 2005-0277727. Thedry emulsions have an outside shell and an internal solid or liquidphase; they form aqueous suspensions or dispersions when mixed withwater.

Solid IFCMB's (such as polymers and waxes) may be manufactured invarious solid shapes, including, but not limited to fibers,approximately spherical beads, films, ribbons and platelets. However,since they must be small enough to enter formation pores, andmanufactured small particles are generally approximately sphericalbeads, then these are the preferred form. When slurried in water, thesematerials could be considered to be a latex, dispersion, or suspension.Sometimes, the particles, for example of a latex, dispersion, orsuspension, will consist only of the IFCMB material itself (for examplea wax or polymer). Sometimes, as part of the process of making theparticles, materials will be used such as surfactants or other polymers;often, these other materials will also be breakers, especially forVES's.

If their degradation is by hydrolysis, the IFCMB's may be coated to slowthe hydrolysis. Suitable coatings include polycaprolate (a copolymer ofglycolide and epsilon-caprolactone), and calcium stearate, both of whichare hydrophobic. Polycaprolate itself slowly hydrolyzes. Generating ahydrophobic layer on the surface of the IFCMB's by any means delays thehydrolysis. Note that coating here may refer to conventionalencapsulation, for example with a polymer, or simply to changing thesurface by chemical reaction or by forming or adding a thin film ofanother material.

Micelles (of any shape, including vesicles) in a VES base fluid can bethe carrier of a hydrophobic component, which will be incorporated intothe hydrophobic core of the micelle. The hydrophobic component can beeither a VES breaker or a breaker activator; examples include linearalcohols and hydrotrops. The release of these components is controlledby diffusion out of the core into the solvent. Thus the componentsrequire a minimum level of water solubility. When vesicles are used forencapsulation of a breaker or activator, the vesicles are most commonlyprepared using a mixed surfactant system containing anionic and cationicsurfactants. Biological vesicles such as liposomes can be used assuitable delivery mechanisms. Emulsions are well known. Encapsulatedbreakers of these types are prepared in water or brine by mechanicalmixing and stabilization of suitable “capsule” (micelle, vesicle,droplet, etc.) sizes by means of mixtures of non ionic, anionic,cationic, and zwitterionic surfactants, and block copolymers. Theviscosity of the organic phase is controlled by the addition of varyingamounts of polymer or oligomer. Many of the polymers previouslydiscussed as IFCMB's (for example PLA, PGA, non crosslinked polyesters,such as alkyd resins, non crosslinked melamine resins, non crosslinkedepoxy resins, non crosslinked phenolic resins, polyvinyl acetatecopolymers, polyacrylate copolymers, for example those acrylatesobtained from long chain alcohols, may be used to control the viscosity.The polymer or oligomer itself, or its degradation products may also beused as effective gel breakers. This approach helps control the rate ofdiffusion of the organic compound out of the capsule and therefore aidsthe slow release of the organic molecule. Breakers that may beencapsulated in micelles, vesicles, or droplets include but are notlimited to: long chain alcohols (for example from about C₈ to aboutC₂₄), aliphatic or aromatic solvents, fatty acids (for example from C₁₂to about C₂₄), fatty acid esters, mono, di or triglicerides of fattyacids, long chain alcohol phosphate esters, diacid esters (also known asdibasic esters) such as dialkyl maleates, dialkyl fumarates, dialkylglutarates, and dialkyl adipates, in which the term alkyl indicatesaliphatic or aromatic groups coming from alcohols having 1 to about 24carbon atoms, for example methyl, ethyl, butyl, dodecyl, octyl,hexadecyl, oleyl, or erucyl alcohols, other dibasic esters, and mixturesthereof.

Enzymes and proteins may be encapsulated in, for example, fatty acids,polyvinyl alcohol and synthetic resins. The solubility and/or theporosity of the coating dictate the time dependent release of theenzyme. Suitable materials are described in copending application“Internal Breaker for Oilfield Fluids,” inventors Philip Sullivan, YiyanChen, Belgin Baser, Carlos Abad, Mehmet Parlar, and Gregory Kubala,filed Sep. 18, 2006.

In use, small amounts of an encapsulated embodiment of an IFCMB, such asa latex containing at least one type of, for example, an organicmolecule that can disrupt the ordering of a viscoelastic surfactantwormlike micelle, is added to a VES fluid system before the fluid isinjected. The subsequent slow release of the organic molecule controlsthe timing of the breaking of the VES gel. The release is controlled byeither the solubility of the latex shell in the aqueous fluid or by theability of the breaker to diffuse through the shell into the base fluid,or by contact of the latex particle surface with the micelle to bebroken. Other parameters that may affect the rate of release of thebreaker are the temperature and the viscosity of the base fluid.

In some applications a slow release is not appropriate, and a delayedrelease is preferred. In such cases degradable surfactants may be usedto stabilize the latex particles; their degradation rate then affectsthe stability of the latex. Examples of degradable surfactants may befound in U.S. Pat. No. 7,036,985, and U.S. Patent Application No.2004-0102330, each of which is assigned to the assignee of the presentapplication and each of which is hereby incorporated in its entirety.

Microbial cells of different species such as alcaligenes latus,alcaligenes eutrophus, bacillus megaterium or ralstonia eutropha, mayunder predetermined growth conditions contain various amounts ofnaturally occurring polyesters, generally known aspolyhydroxyalkanoates, such as copolymers of polyhydroxybutyrate,polyhydroxyvalerate, polydroxyhexanoate, and/or polyhydroxyoctanoate.Bacteria containing these polymers may be used to transport delayedbreakers. The conditions downhole (for example pH, temperature,pressure, and salinity) may be used to cause the bacterial cell membraneto destabilize, rupture or burst by mechanisms such as osmosis. Thesebacteria are then acting to encapsulate the polyesters, which may bebreakers of VES fluids when they are released when the cell membranebursts. Additional delay is possible due to the kinetics of thedecomposition of the polyesters. Compounds such as waxes, oligomers, orother polymers that are not assimilated by living cells, or bioproductsthat can be assimilated by living cells (such as fat or mono, di ortriglicerides), but that are stored in the cytoplasm when there is anexcess of nutrients and a deficit of critical compounds for theirmetabolism (for instance lack of a required oligoelement such as ametal, or a particular aminoacid), and are VES breakers, can beincorporated into the cytoplasm of such cells. This is calledendocytosis when a cell engulfs some of its intracellular fluid; it isphagocytosis when the engulfed material is suspended particles andpinocytosis when the engulfed material had been in solution. Thecontents of cells are protected from their environments by variousbiological membranes. These membranes are permeable and can be used toallow slow release of the contents of the cell by diffusion; thecontents of the cells may also be liberated by bursting of the cellmembrane by changes in temperature, pressure, pH, or salinity. Thus thecells containing the appropriate chemicals may be delayed VES breakers.The compounds forming the cell membrane can also be liberated during thebursting process, and eventually become breakers for the fluid. Thecontents of the microbial cells themselves is composed of multipleorganic compounds (polysaccharides, lipids, nucleic acids, and notablyproteins of all the types that may also be breakers for VES fluids.

In some cases it may be necessary or suitable to accelerate thedegradation of IFCMB's. This may be done in many ways, depending uponthe chemistry and form of the IFCMB. Accelerants may be incorporated inthe IFCMB particles or may be in the fluid phase containing the IFCMB'sor may be in another fluid phase or in another fluid. More than one typeof accelerant may be used. One type of IFCMB may be an accelerant foranother type of IFCMB. Non-limiting examples of methods of acceleratingthe degradation of IFCMB's include further including oxidizers, with orwithout oxidation catalysts; further including latent acids; furtherincluding enzymes; and by increasing the temperature.

The IFCMB's may be added to a wellbore fluid by metering them in to thebase water fluid as a concentrated liquid. If the material is receivedas a suspension or dispersion, it can be stored in that form and used inthat form directly. If it is received in dry form (for example as asolid dispersible powder of fine polymer beads or as a dry emulsion) theparticles can be pre-dispersed in water or brine as required and meteredin as a liquid stream, or alternatively they may be added as solids tothe base fluid stream. Note that mixtures of different types of IFCMB'smay be used, including mixtures of different chemistries (for example amixture of two different latexes, or a mixture of a wax and a latex) andmixtures of two different physical forms (for example a mixture of anemulsion containing one type of particle in the discontinuous phase andone type of particle in the continuous phase).

The IFCMB degrades (self-destructs) in situ, that is, in the locationwhere it is placed. Although we have discussed here primarily oilfieldtreatment uses in which the material has been placed inside formationpores, the material may also be placed as part of a suspension in atreatment fluid in the wellbore, in the perforations, in a gravel pack,or in a fracture; or as a component of a filter cake on the walls of awellbore or of a fracture. It is particularly advantageously used whereit is difficult to introduce a breaker (for example in a kill pill) orwhere it is difficult to trigger a break (for example in a gravel pack).The IFCMB may also be used in other industries such as household andindustrial cleaning.

A particular advantage of many these materials is that they and theirdegradation products are generally not toxic to humans and aquaticanimals and are they are typically biodegradable.

The IFCMB's may be used in carbonates and sandstones. In carbonates,acid produced by hydrolysis of some IFCMB's stimulates the formation.Such mixtures that generate extra acid are particularly useful indrilling, “drill-in”, and stimulation operations in carbonatereservoirs.

The rate of degradation of a particular IFCMB, having a particularchemical and physical make-up, including a coating if present, at aparticular temperature and in contact with a viscosified fluid or fluidsof a particular composition (for example pH and the concentration andnature of other components, especially electrolytes), is readilydetermined by a simple experiment: exposing the IFCMB to the fluid orfluids under treatment conditions and monitoring the viscosity.

Some VES systems, for example some cationic systems, are not verysensitive to pH, and some VES systems, for example some anionic systems,are typically buffered to a pH of above 12 in normal use, and theIFCMB's of this Invention that generate acid upon degradation may not beeasily used as breakers of such systems. In those cases, other IFCMB'sare used.

The IFCMB's of this Invention may be used with VES's made with any typeof surfactant, or mixtures of surfactants, with or without one or moreco-surfactants, and with or without other additives intended tostabilize or modify the properties of the micelles or vesicles (such asbuffers, shear recovery additives, salts, and rheology boosters).Preferred VES's are cationic, anionic, amphoteric, and zwitterionic.Suitable VES's, for example, are described in the following U.S.Patents, all of which are hereby incorporated in their entirety: U.S.Pat. Nos. 5,964,295; 5,979,557; 6,306,800; 6,637,517; and 6,258,859. Theviscoelastic surfactant may be, for example, of the following formulae:R-Z, where R is the hydrophobic tail of the surfactant, which is a fullyor partially saturated, linear or branched hydrocarbon chain of at least14 carbon atoms and Z is the head group of the surfactant which may befor example —NR₁R₂R₃ ⁺, —SO₃ ⁻, —COO⁻ or, in the case where thesurfactant is zwitterionic, —N⁺(R₁)(R₂)R₃—COO⁻ where R₁, R₂ and R₃ areeach independently hydrogen or a fully or partially saturated, linear orbranched, aliphatic chain of at least one carbon atom; and where R₁ orR₂ may comprise a hydroxyl terminal group.

Cleavable viscoelastic surfactants, for example of the followingformula, may be used, as disclosed in International Patent ApplicationWO02/064945: R-X-Y-Z, where R is the hydrophobic tail of the surfactant,which is a fully or partially saturated, linear or branched hydrocarbonchain of at least 18 carbon atoms, X is the cleavable or degradablegroup of the surfactant which is an acetal, amide, ether or ester bond,Y is a spacer group which is a short saturated or partially saturatedhydrocarbon chain of n carbon atoms where n is at least equal to 1,preferably 2 and, when n is equal to or greater than 3, the chain may bea straight or branched saturated or partially saturated chain, and Z isthe head group of the surfactant which can NR₁R₂R₃ ⁺, —SO₃ ⁻, —COO⁻ or,in the case where the surfactant is zwitterionic, —N⁺(R₁R₂R₃—COO⁻) whereR1, R2 and R3 are each independently hydrogen or a fully or partiallysaturated, linear or branched, aliphatic chain of at least one carbonatom, possibly comprising a hydroxyl terminal group. Due to the presenceof the cleavable or degradable group, cleavable surfactants are able todegrade under downhole conditions.

A nonlimiting example of a suitable cationic viscoelastic surfactantuseful for the implementation of the Invention isN-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride. Nonlimitingexamples of some suitable anionic viscoelastic surfactants useful forthe implementation of the Invention are monocarboxylates RCOO— such asoleate where R is C₁₇H₃₃ or di- or oligomeric carboxylates such as thosedisclosed in International Patent Application WO 02/11874.

The IFCMB's of this Invention have been found to be particularly usefulbreakers when used with several types of zwitterionic surfactants. Ingeneral, suitable zwitterionic surfactants have the formula:RCONH—(CH₂)_(a)(CH₂CH₂O)_(m)(CH₂)_(b)—N⁺(CH₃)₂—(CH₂)_(a),(CH₂CH₂O)_(m),(CH₂)_(b),COO⁻in which R is an alkyl group that contains from about 11 to about 23carbon atoms which may be branched or straight chained and which may besaturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and mand m′ are each from 0 to 13; a and b are each 1 or 2 if m is not 0 and(a+b) is from 2 to about 10 if m is 0; a′ and b′ are each 1 or 2 when m′is not 0 and (a′+b′) is from 1 to about 5 if m is 0; (m+m′) is from 0 toabout 14; and CH₂CH₂O may also be oriented as OCH₂CH₂. Preferredsurfactants are betaines and amidoamine oxides.

Two examples of betaines are oleylamidopropyl dimethyl betaine anderucylamidopropyl dimethyl betaine. Oleylamidopropyl dimethyl betainecontains an oleyl acid amide group (including a C₁₇H₃₃ alkene tailgroup); erucylamidopropyl dimethyl betaine contains an erucic acid amidegroup (having a C₂₁H₄₁ tail group). Betaine surfactants, and others thatare suitable, are described in U.S. Pat. No. 6,258,859.

Although the Invention has been described throughout using the term“VES”, or “viscoelastic surfactant” to describe the non-polymericviscosified aqueous fluid, any non-polymeric material may be used toviscosify the aqueous fluid provided that the requirements describedherein for such a fluid are met, for example the required viscosity,stability, compatibility, and lack of damage to the wellbore, formationor fracture face. Examples, without regard to whether they form, or aredescribed as forming, vesicles or viscoelastic fluids, include, but arenot limited to, those viscosifiers described in U.S. Pat. No. 6,035,936and in GB application No. 2,366,307A.

Also optionally, fracturing fluids may contain materials, such asfibers, designed to assist in proppant transport and/or to limitproppant flowback after the fracturing operation is complete by forminga porous pack in the fracture zone. Such materials can be any known inthe art, such as are available from Schlumberger under the tradenamePropNET™ (for example see U.S. Pat. No. 5,501,275). Exemplary proppantflowback inhibitors include fibers or platelets of novoloid ornovoloid-type polymers (U.S. Pat. No. 5,782,300).

The IFCMB's of the Invention may also be used in gelled oils such as agelled refined oil (commonly called a “frac oil” in the industry) or anysimilar material that provides some viscosity and does not leavedamaging solids in the fracture. The following products of DomePetroleum Limited of Calgary, Alberta, may be used: FRAC OIL 120, FRACOIL 200, FRAC OIL 300, FRAC OIL 500; as well as SUPER FRAC™ availablefrom Home Oil Company Limited of Calgary; as well as kerosene, dieselfuel, condensate, and crude oil. The IFCMB may not be a breaker forgelled oils.

Some of the IFCMB's of the Invention may also be used in fluidsviscosified with polymers such as biopolymers, such as polysaccharides,including crosslinked polymers, provided, of course, that the IFCMBselected contains or releases an agent, such as an acid, a chelatingagent, or an oxidizer, that will break either the polymer or thecrosslink or both.

The particle size of the IFCMB is chosen based primarily on the desiredfluid loss properties. If an external filter cake is desired, at least aportion of the IFCMB must be too large to enter the pores of the matrix.If an external filter cake is not desired, the particles must be smallenough to enter the pores. Then the size depends upon whether aninternal filter cake is desired (which would be the case, for example ifa filter cake was desired but an external filter cake would be subjectto erosion). The choice of material is based primarily on the desiredrate of degradation in the fluid to be used at the temperature at whichit will be used. These choices may also be influenced by the desiredtime before the delayed break, which could depend upon the size of thejob, the nature of the job, and other factors known to those of ordinaryskill in the art. Similarly, the concentration of any buffer, delayagent, or accelerating agent is based upon many factors that will beclear to one of ordinary skill in the art, including the concentrationsand natures of the VES, the IFCMB and any other additives, thetemperature, and the desired time to break. The appropriate buffer,delay agent, or accelerating agent concentration can be determined bysimple laboratory experiments, for example mixing all the components,heating to the job temperature, and monitoring the viscosity. The systemcomprising an IFCMB may be used in any aqueous fluid from fresh water toheavy brines; a requirement is compatibility of the water with the VESsystem. The system comprising an IFCMB also works with VES systems thatcontain co-surfactants or other additives commonly included in oilfieldtreatment fluids. Again, a requirement is compatibility with the VESsystem. The fluid containing an IFCMB may be batch-mixed or mixedon-the-fly.

When IFCMB's are used in fluids in such treatments as drilling,drill-in, completion, stimulation (for example, hydraulic fracturing ormatrix dissolution), sand control (for example gravel packing,frac-packing, and consolidation), diversion, well killing, lostcirculation control, and others, the IFCMB's are initially inert to theother components of the fluids, so the other fluids may be prepared andused in the usual way. When Resin Coated Proppants (RCP's) are used,testing should be done to ensure that the RCP's and IFCMB's arecompatible and that neither interferes with the performance of theother; conventional natural and synthetic proppants and gravels maynormally be used without testing. Normally, such fluids already containa fluid loss additive and filter cake former, so the IFCMB's replacesome or all of the fluid loss additive and filter cake former that wouldotherwise have been used. In many cases, if the fluid contains acomponent that would affect or be affected by the IFCMB (such as abuffer, an acid-reactive material, or a viscosifier that forms or isincorporated in filter cakes), either the amount or nature of the IFCMBor the amount or nature of the interfering or interfered-with componentmay be adjusted to compensate for the interaction. This may readily bedetermined by simple laboratory experiments.

Any additives normally used in such treatments may be included, againprovided that they are compatible with the other components and thedesired results of the treatment. Such additives can include, but arenot limited to anti-oxidants, crosslinkers, corrosion inhibitors, delayagents, biocides, buffers, fluid loss additives, etc. The wellborestreated can be vertical, deviated or horizontal. They can be completedwith casing and perforations or open hole.

In gravel packing, or combined fracturing and gravel packing, it iswithin the scope of the Invention to apply the compositions and methodsof the Invention to treatments that are done with or without a screen.Although treatments are normally done to promote hydrocarbon production,it is within the scope of the Invention to use the compositions andmethods of the Invention in wells intended for the production of otherfluids such as carbon dioxide, water or brine, or in injection wells.Although we have described the Invention in terms of unfoamed fluids,fluids foamed or energized (for example with nitrogen or carbon dioxideor mixtures thereof) may be used. Adjustment of the appropriateconcentrations due to any changes in the fluid properties (or otherparameters, such as proppant concentration) consequent to foaming wouldbe made.

EXPERIMENTAL Example 1

Waxes

As with other materials suitable as IFCMB's, when wax particles aresignificantly smaller that the formation pore sizes, then the waxparticles act only as sources of a breaker. In this case, they release abreaker for most viscoelastic surfactant based fluid systems when theymelt. When the particles are larger relative to the pore throats of theformation, they form an internal filter cake, and then when they meltthey provide the breaker. One skilled in the art will know how to choosethe appropriate size range that will or will not form an external orinternal filter cake for a given formation, and will know how to test todetermine the proper wax particle concentration.

FIG. 1 shows the viscosity as a function of time at 65.6° C. (150° F.)of a fluid viscosified with 4 weight percent of a viscoelasticsurfactant concentrate containing about 38 weight % erucylamidopropyldimethyl betaine surfactant, 1.1 weight % polynaphthalene sulfonate, 22weight % isopropanol, 5 weight % sodium chloride and the remainderwater. The fluid also contained 2 weight % KCl and 1 weight %ethyleneglycol monobutyl ether. The figure shows the viscosity of thisfluid with no wax, 1 weight % wax dispersion, and 2 weight % waxdispersion. The wax was “paraffin 150-50” obtained from ChemCor,Chester, N.Y., U.S.A. This material is a highly refined petroleumproduct containing less than 3 % oil, and is a low molecular weight,straight chain hydrocarbon with a melting point of about 66° C. (about150° F.). It is used in commercial car washes. As received, it is calledan “emulsion” by the manufacturer and is about 40-51% wax and has a pHof about 9.7 to 10.7. It can be seen that this fluid at this temperatureshowed no tendency to break in the absence of the wax. The waxdispersion caused a break; with increasing amounts of wax dispersion,the rate of break and extent of break both increased.

FIG. 2 shows the break in viscosity of fluids containing 4 weight % or 8weight % of the same viscoelastic surfactant concentrate in a fluidcontaining 2 weight % KCl, 1 weight % ethyleneglycol monobutyl ether,and 1 weight % of the same wax dispersion at 65.6° C. (150° F.). In thiscase, the fluid containing the higher VES concentration was stable formany hours.

FIG. 3 shows the fluid containing 4 weight % of the same viscoelasticsurfactant concentrate, 2 weight % KCl, 1 weight % ethyleneglycolmonobutyl ether, and 1 weight % of the same wax dispersion at varioustimes and various shear rates at 65.6° C. (150° F.). The results wereobtained with a Bohlin rheometer. In these experiments, 5 hours wasrequired to reduce the viscosity to about 10 cp over the full shear raterange examined.

FIG. 4 shows a fluid containing 6 weight % of the same viscoelasticsurfactant concentrate, 1.38 kg/L (11.5 ppg) NaBr, 1 weight %ethyleneglycol monobutyl ether, and 1 weight % of the same waxdispersion at various times and various shear rates (Bohlin rheometer)at 65.6° C. (150° F.). The fluid shows faster initial viscosity lossesat low shear rates than at high shear rates, but shows useful viscosityat low shear rates for at least 5 days and for at least 4 days at highshear rates. This system could be described as displaying a controlledbreak at about 5 days.

FIG. 5 shows the same experiment as that of FIG. 4, except that theadded salt was CaBr₂. The fluid was less stable than that with NaBr.FIG. 6 shows the same experiment as those of FIGS. 4 and 5, except thatthe salt was CaCl₂. The fluid has a higher viscosity and is more stablethan the fluids than CaBr₂ or NaBr. In heavy brines, the break can becontrolled further by the choice of the weighting salt.

Not shown are data from an experiment done with another wax dispersion,“polyethylene 629A30” also obtained from ChemCor, Chester, N.Y., U.S.A.,and also called a wax “emulsion” by the manufacturer. The polyethylenewas low-density, had a melting point of 104° C., had been made anionicby copolymerization with acrylic acid, to aid in the emulsification, andwas in the sodium form. In experiments similar to those described above,this material did not break the viscoelastic surfactant, even above itsmelting point.

Example 2

Polyesters

That carboxylic acids released by the degradation of polyesters are VESbreakers is demonstrated in FIG. 7, which shows measurements of theviscosity of one example of a VES fluid system with different amounts ofglycolic acid (GA) dissolved in the fluid. Measurements were conductedat 66° C. (150° F.). The buffer used in the experiments shown in FIG. 7was sodium sesquicarbonate, which was used at a concentration of 30pounds per thousand gallons (3.6 g/L) in all experiments. The VES fluidsystem was made with 6% of the same viscoelastic surfactant concentrateas was used in Example 1. This experiment showed the results that wouldbe seen as polyglycolic acid (PGA) degrades, and demonstrates thebreaking effect on the fluid viscosity as the PGA hydrolyzes to formglycolic acid. In addition, the data in the figure also demonstrate thatthe viscosity of this VES is not significantly reduced by the additionof a pH buffer to maintain the fluid pH at approximately 9.5.

It can be seen that the viscosity of the surfactant system with nobuffer or PGA (top line, diamonds) was reduced by the addition of 0.5%PGA (42 pounds per thousand gallons, or 0.5 g/L), and the pH had alreadygone down to 4 when it was measured. The PGA used was DuPont TLF 6267™,which may contain up to about 5% glycolic acid as received, and about90% of which has a particle size of less than about 20 microns. Thismaterial is a crystalline PGA with a molecular weight of about 600. Tosimulate hydrolysis and dissolution of PGA, increasing amounts of GAwere added to portions of the baseline fluid system; this resulted insuccessively greater decreases in the viscosity. The viscosity of thebaseline material was not affected by the addition of sodiumsesquicarbonate to buffer the system at a pH of approximately 9.5.

FIG. 8 shows the viscosity of the same 6% surfactant fluid system as thebaseline system of the experiments shown in FIG. 7, determined with aFann 50 Viscometer over a range of temperatures, with PGA added, withand without the pH 9.5 buffer. Without the buffer in place, theviscosity of the fluid was substantially reduced; therefore this PGAalone would have been unsatisfactory as an internal filter cake formerand/or as a matrix breaker, except for the shortest of oilfieldtreatments. With the buffer present, the viscosity of the fluid systemcontaining the PGA as an FLA and delayed breaker was essentiallyidentical to the viscosity of the baseline system. The total duration ofeach of these experiments was about 3 hours. At the end of the run withthe buffer it can be seen that the viscosity dropped below the baseline,suggesting that the hydrolysis of the PGA at the higher temperature wasstarting to break the fluid when the temperature was above about 121° C.(250° F.). (The buffer was being overwhelmed at this point.) The timethat this system was at about 121° C. (250° F.) was about 160 minutes.Therefore, this fluid system, containing PGA as an FLA and delayedbreaker, is suitable for use in hydraulic fracturing and gravel packing.

Example 3

Dry Emulsions

The dry emulsion used was obtained from Rhodia, Inc. Cranbury, N.J.,U.S. A. and was made by preparing a dispersion of an oil phase in waterusing a hydrophilic polymer as the shell-polymer, and then drying theresulting emulsion to form a dry solid. Such dry emulsions are usuallystabilized with a surfactant and/or with a hydrophilic polymericstabilizer. Such dry emulsion materials are a very convenient method ofencapsulating organic compounds that are VES breakers, by dissolvingthem in the oil phase before formation of the dry emulsion. The choiceof oil solvent type and amount, surfactant type and concentration,hydrophilic polymer type and amount, and VES breaker type andconcentration in the oil, all have an impact on the particle size of the“dried emulsion”, as well as on the extent of the break, and the delay,when the IFCMB is used to break a VES fluid system. All of theseselections depend upon the required conditions for the specific oilfieldapplication. It should be noted that the emulsion, before drying, mayalso be used as an IFCMB.

In experiments not shown, the viscosity of a fluid containing 6 weight %of the same viscoelastic surfactant concentrate used in the previousexamples in a 1.39 kg/L (11.6 ppg (pounds per gallon)) NaBr brine wasfollowed in a Fann 50 Rheometer at 65.6° C. (150° F.) for up to 14hours. With no added breaker, the viscosity was constant at about 40 cPat 100 sec⁻¹. When 1% by weight of the dry emulsion made with the oilhad been added, the viscosity was still constant. When 2% by weight ofthe same dry emulsion had been added, the viscosity began to decreaseafter about 7 hours and was similar to that of water after about 12hours.

FIG. 9 shows the viscosity vs. shear rate from the day of preparation tothe tenth day of storage at 65.6° C. (150° F.) of a betaine viscoelasticsurfactant fluid system containing a dry emulsion internal filter cakeformer/matrix breaker of the Invention. The fluid contained 6 weight %of the same viscoelastic surfactant concentrate used in the previousexamples in a 1.38 kg/L (11.5 ppg (pounds per gallon)) NaBr brine with 1weight % of the dry emulsion made with the paraffin oil. The viscositywas measured at 65.6° C. (150° F.) with a Bohlin rheometer. The lowshear viscosity declined steadily over the ten days to about 1/1000 ofits original value but it was still significant at the end of theexperiment. The high shear viscosity was constant for at least the first5 days, but had decreased substantially after 10 days.

FIG. 10 shows the viscosity vs. shear rate from the day of preparationto the tenth day of storage at 65.6° C. (150° F.) of a betaineviscoelastic surfactant fluid system containing a dry emulsion internalfilter cake former/matrix breaker of the Invention. The fluid contained6 weight % of the same viscoelastic surfactant concentrate used in theprevious examples in a 1.38 kg/L (11.5 ppg (pounds per gallon)) CaCl₂brine with 1 weight % of the dry emulsion made with the paraffin oil.The viscosity was measured at 65.6° C. (150° F.) with a Bohlinrheometer. The behavior was similar to that observed in FIG. 9, exceptthat the high shear viscosity decreased somewhat during the first 5 daystoo.

FIG. 11 shows the viscosity vs. shear rate from the day of preparationto the tenth day of storage at 65.6° C. (150° F.) of a betaineviscoelastic surfactant fluid system containing a dry emulsion internalfilter cake former/matrix breaker of the Invention. The fluid contained6 weight % of the same viscoelastic surfactant concentrate used in theprevious examples in a 1.38 kg/L (11.5 ppg (pounds per gallon)) CaBrbrine with 1 weight % of the dry emulsion containing a paraffin oil. Theviscosity was measured at 65.6° C. (150° F.) with a Bohlin rheometer.The low shear behavior was similar to that observed in the previous twoexperiments, but the decrease in viscosity at intermediate and highshear rates occurred more uniformly over the entire time period.

FIG. 12 shows the viscosity vs. shear rate from the day of preparationto the tenth day of storage at 65.6° C. (150° F.) of a betaineviscoelastic surfactant fluid system containing a dry emulsion internalfilter cake former/matrix breaker of the Invention. The fluid contained4 weight % of the same viscoelastic surfactant concentrate used in theprevious examples in a 1.38 kg/L (11.5 ppg (pounds per gallon)) NaBrbrine with 1 weight % of the dry emulsion containing a paraffin oil. Theviscosity was measured at 65.6° C. (150° F.) with a Bohlin rheometer.There was little loss of low shear viscosity until after the fifth dayand little loss of high shear viscosity until the fifth day.

1. A method of treating a subterranean formation penetrated by awellbore comprising a formation surface comprising a) injecting into theformation through the formation surface a fluid comprising a viscosifierand a solid additive comprising particles sufficiently small to passinto the formation pores, said solid additive comprising a materialdegradable in the formation, and b) allowing said additive to degradeinto a material soluble in a fluid in the pores after the injection. 2.The method of claim 1 wherein said material is degradable by a mechanismselected from thermal degradation, melting, hydrolysis and anycombination of thermal degradation, melting, and hydrolysis.
 3. Themethod of claim 1 further comprising allowing fluid to flow between theformation and the wellbore or between the wellbore and the formation. 4.The method of claim 1 wherein the particles form an internal filtercake.
 5. The method of claim 2 wherein the particles further form anexternal filter cake.
 6. The method of claim 1 wherein the particles donot form an external filter cake.
 7. The method of claim 1 wherein thesoluble material is a breaker for the viscosifier.
 8. The method ofclaim 1 wherein the step of injection is carried out above the formationfracture pressure to form one or more fractures having faces.
 9. Themethod of claim 1 wherein the fluid further comprises proppant orgravel.
 10. The method of claim 1 wherein the fluid further comprises aformation dissolving compound.
 11. The method of claim 1 wherein theviscosifier comprises a viscoelastic surfactant.
 12. The method of claim11 wherein the viscoelastic surfactant is selected from the groupconsisting of quaternary amines, betaines, carboxylic acids, andamidoamine oxides.
 13. The method of claim 1 wherein the viscosifiercomprises a polysaccharide.
 14. The method of claim 13 wherein theviscosifier comprises a polysaccharide and the particles comprise amaterial selected from the group consisting of polyesters,polycarbonates, polyacetals, polymelamines, polyvinyl chlorides,polyvinyl acetates, polyvinyl esters, polyacrylates, polymethacrylates,polycarboxylates and mixtures thereof.
 15. The method of claim 1 whereinthe particles comprise polyesters.
 16. The method of claim 1 wherein theparticles comprise a material selected from the group consisting ofwaxes, polycarbonates, polyacetals, polymelamines, polyvinyl chlorides,polyvinyl acetates, polyvinyl esters, polyacrylates, polymethacrylates,polycarboxylates, and mixtures thereof.
 17. The method of claim 16wherein the fluid further comprises a pH control agent present in anamount sufficient to neutralize any acid or base present in the solidmaterial before the injection and any acid or base generated by thesolid material during the injection.
 18. The method of claim 1 whereinthe viscosifier comprises a viscoelastic surfactant and the particlescomprise a material selected from the group consisting of waxes,polyesters, polycarbonates, polyacetals, polymelamines, polyvinylchlorides, polyvinyl acetates, polyvinyl esters, polyacrylates,polymethacrylates, polycarboxylates, and mixtures thereof.
 19. Themethod of claim 1 wherein the fluid further comprises a fluid lossadditive comprising particles too large to pass into the formationpores.
 20. The method of claim 1 wherein said solid additive particlesdo not form a substantially water impermeable film on formation orfracture faces.
 21. The method of claim 1 wherein the particles areprovided as a suspension.
 22. The method of claim 1 wherein theparticles are provided as a dispersion.
 23. The method of claim 1wherein the particles are provided as a latex.
 24. The method of claim 1wherein the particles are provided in the internal phase of anoil-in-water emulsion.
 25. The method of claim 1 wherein the particlesare provided inside structures selected from the group consisting ofmicelles, liposomes, vesicles, eucariote and procariote cells, andbacteria.
 26. The method of claim 25 wherein the particles are providedinside micelles.
 27. The method of claim 25 wherein the particlescomprise a member selected from the group consisting of long chainalcohols, aliphatic or aromatic solvents, fatty acids, fatty acidesters, mono, di or triglicerides of fatty acids, long chain alcoholphosphate esters, diacid esters, and mixtures thereof.
 28. A fluidcomposition comprising a viscosifier and a solid additive comprisingparticles sufficiently small to enter subterranean formation pores, saidsolid additive comprising a material degradable in the formation into amaterial soluble in a fluid in the pores of the formation and capable ofbreaking the viscosifier.